1. Field of the Invention
The invention relates generally to the field of electronic circuits, and more particularly, to enhancing slew rates in electronic circuits such as amplifier circuits.
2. Background Information
The present invention relates generally to high speed equipment such as communication systems, displays, and the like, ranging from cell phones to computer displays, in which the large signal behavior of the amplifier is an important consideration. The present invention involves a variety of designs which may provide techniques to achieve good large signal behavior in a variety of circuit arrangements.
Analog circuit elements in a system need to apply a specified signal swing to the next element in the circuit. The next element often has a required signal swing for maximum dynamic range, such as the full scale level of an analog to digital converter. As systems evolve for use at higher bandwidths, analog circuit elements, particularly elements with feedback, reach their large signal limits. Large signal performance is usually measured as slew rate, defined as the maximum rate of voltage change possible for a circuit at a given node.
The classic slew rate limit in an amplifier often occurs when a stage in the amplifier (usually an input stage) has a fixed maximum current, and this stage must charge capacitance. This is illustrated in FIG. 1, which shows an amplifier circuit 10 that includes an input stage 20 and an output stage 30. Input stage 20 supplies a current with a fixed maximum to output stage 30, and this current can be applied to a capacitor 104 that resides in output stage 30.
As shown in FIG. 1, input stage 20 includes a differential pair of transistors 101 and 102 that receive a differential input signal 111 at their respective base terminals 110 and 112. Differential input signal 111 controls how much current flows through transistors 101 and 102. A current generator 103 is coupled to transistors 101 and 102 at their respective emitter terminals 114 and 116. Current generator 103 provides amplifier circuit 10 with current, and this current typically has a fixed maximum. Therefore, the maximum current that transistors 101 and 102 can apply to output stage 30, and ultimately to capacitor 104, is the current generated by current generator 103. This can then determine the maximum charging rate of capacitor 104 by the current-voltage relationship for a capacitor:                     ∂        V                    ∂        t              =          I      C        ⁢      xe2x80x83  
where       ∂    V        ∂    t  
is the time rate of change of voltage (slew rate), I is the current 103 and C is the capacitance 104.
To complete the circuit of input stage 20, transistors 101 and 102 include a pair of collector terminals 118 and 120 that are electrically coupled to a pair of resistors 105 and 106. Resistors 105 and 106 are in turn coupled to a path to ground 122.
Output stage 30 includes a transistor 107 with an emitter terminal 124 that is electrically coupled to collector terminal 120 of transistor 102. Transistor 107 is operated by a reference voltage 129 that is applied to a base terminal 126 of transistor 107. Transistor 107 also has a collector terminal 128 that is coupled to capacitor 104 via output line 132, and capacitor 104 is in turn coupled to a path to ground 130. Output line 132 is typically connected to additional circuitry that is unrelated to amplifier circuit 10 for the purposes of this description, and is therefore not shown.
While there are a variety of techniques in use to improve slew rate, no single design achieves its goals without limiting performance in other areas. For instance, a common technique well known in the art is input stage degeneration. This technique does provide larger slew rates, but it increases noise and degrades open loop gain. Another common technique uses an input stage which is not current limited, one example of which is set forth in U.S. Pat. No. 5,049,653 issued to Smith, et al., which is hereby incorporated by reference. This technique is commonly used in current feedback amplifiers, and has been implemented in voltage feedback amplifiers as well. The drawback of a non-current limited input stage is that more voltage is required to bias the stage, making it not usable in low supply voltage or battery applications.
In addition, it is not feasible to use a non-current limited input stage in single supply applications which require the input common mode range to include one or both supplies. Thus, even though solutions to the problems mentioned in this disclosure have existed, none are believed to have provided the proper balance of competing concerns in most applications and certainly none have met the various criteria which may now be met by the present invention, especially in the low voltage, single supply or battery operated area or the like. Accordingly, there is a need for an improved amplifier system and method that does not limit the slew rate of a circuit.
The disadvantages and problems associated with current limited stage amplifiers and other similar circuits have been improved using the present invention.
In accordance with an embodiment of the invention, an amplifier circuit comprises an input stage capable of receiving and amplifying an input signal, a gain stage capable of further amplifying the input signal, wherein the gain stage is electrically coupled to the input stage, and an output stage capable of charging a capacitance of the amplifier circuit and outputting the amplified input signal, wherein the output stage is electrically coupled to the gain stage.
According to another embodiment of the invention, the input stage of the amplifier circuit comprises a differential pair of transistors, each transistor comprising a base, an emitter, and a collector, a current generator electrically coupled to the emitters of the differential pair of transistors, a pair of input lines electrically coupled to the bases of the differential pair of transistors, the input lines configured to carry an input signal, a pair of resistors electrically coupled to the collectors of the differential pair of transistors, and a path to ground electrically coupled to the pair of resistors.
According to yet another embodiment of the invention, the gain stage of the amplifier circuit comprises a pair of gain transistors, each gain transistor comprising a base, an emitter, and a collector, wherein the bases of the gain transistors are electrically coupled to the input stage, wherein the collectors of the gain transistors are electrically coupled to a path to ground, and wherein the emitters of the gain transistors are electrically coupled to the output stage.
And according to yet another embodiment of the invention, the output stage of the amplifier circuit comprises a pair of output transistors, each output transistor comprising a base, an emitter, and a collector, wherein the emitters are electrically coupled to the gain stage, a reference voltage line electrically coupled to the bases of the output transistors, the reference voltage line configured to carry a reference voltage signal, and a pair of output lines electrically coupled to the collectors of the output transistors.
An important technical advantage of the present invention includes applying the amplified input signals produced in the input stage of an amplifier circuit to the base terminals of transistors in a gain stage of the amplifier circuit. This configuration breaks the typically direct relationship between the input stage and the output stage of an amplifier circuit. So here, when the low current, input stage signals are disassociated from the output stage of the amplifier circuit, the output stage is free to operate at higher currents. The result is an increase in the slew rate of the amplifier circuit.